This invention relates to a transducer of the vibration type, such as an electromechanical transducer of the Coriolis type which is particularly suited for use in a Coriolis mass flowmeter.
To determine the mass flow rate of a medium flowing in a pipe, particularly of a fluid, use is frequently made of measuring devices which produce Coriolis forces in the fluid by means of an electromechanical transducer of the Coriolis type driven by control and evaluation electronics connected thereto, and which derive from the Coriolis forces a measurement signal representative of mass flow rate.
Coriolis mass flowmeters or Coriolis mass flow/density meters have been known and in industrial use for a long time. US-RE 31,450, for example, discloses a Coriolis mass flow meter with an electromechanical transducer of the Coriolis type which provides an output in response to the mass flow rate of a fluid flowing in a pipe and which comprises:
a single flow tube, curved in a tube plane symmetrically with respect to an axis of symmetry lying in said plane, for conducting the fluid;
a rigid support body for mounting the flow tube,
the flow tube being fixed to the support body by an inlet-side mounting and an outlet-side mounting; and
an excitation system which in operation excites the flow tube into cantilever beam vibrations of a first eigenmode, at which the flow tube is subjected to torsion out of the tube plane about a tube axis which joins the two mountings and is perpendicular to the axis of symmetry.
As is well known, such flow tubes bent in one plane, e.g., U-shaped tubes, if excited into cantilever beam vibrations, cause Coriolis forces in the fluid passing therethrough. These, in turn, result in torsional vibrations about the axis of symmetry according to a second eigenmode of the flow tube being superimposed on the beam vibrations, so that vibrations at the inlet and outlet ends exhibit a measurable phase difference, which is also dependent on mass flow rate.
To determine such a phase difference, the prior-art mass flowmeter further comprises an electrodynamic sensor arrangement which serves to punctually sense vibrations of the flow tube at the inlet and outlet ends and to generate electric sensor signals influenced by the mass flow rate of the fluid.
In operation, the flow tubes of such meters are usually excited at an instantaneous resonance frequency of the first natural mode, particularly with the vibration amplitude maintained constant. Since this resonance frequency is also dependent on the instantaneous density of the fluid, commercially available Coriolis mass flowmeters can also be used to measure the density of the fluid.
Because of the curved tube shape, the flow tubes of such Coriolis mass flowmeters can be made relatively long, so that high sensitivity of the transducer to the mass flow rate to be measured can be achieved with a relatively short mounting length and with relatively low excitation energy. This also permits the flow tube to be made from materials with a high modulus of elasticity, particularly from high-quality steel. In such meters with a straight flow tube, for example, the latter must generally be made of a material having a lower modulus of elasticity than high-quality steel in order to achieve sufficient sensitivity. Therefore, flow tubes of titanium or zirconium are preferably used for such meters, but because of the higher material cost and the usually higher machining cost, such tubes are much more expensive than those made from high-quality steel.
Another advantage of a curved flow tube is that thermally induced expansion, particularly in flow tubes with a high expansion coefficient, produce virtually no or only very slight mechanical stresses in the connected pipe. A known disadvantage of such a design of the Coriolis mass flowmeter is that in operation, inertial forces act via the flow tube, particularly because of the lateral deflections of the tube, and may cause torsional vibrations and/or flexural vibrations in the connected pipe.
To reduce such unwanted effects, commercially available Coriolis mass flowmeters are frequently offered with two identically bent, parallel flow tubes. U.S. Pat. Nos. 4,491,025, 4,768,385, or 5,359,881, for example, discloses an electromechanical transducer of the Coriolis type which provides an output in response to the mass flow rate of a fluid flowing in a pipe and which comprises:
two identical flow tubes for conducting the fluid, each of which is curved in an associated tube plane symmetrically with respect to an associated axis of symmetry lying in this plane;
a rigid support body for mounting the flow tubes,
each of the flow tubes being fixed to the support body by an associated inlet-side mounting and an associated outlet-side mounting; and
an excitation system which in operation excites the flow tubes into cantilever beam vibrations of a first natural mode, in which the flow tubes are subjected to torsion out of their respective planes about a tube axis which joins the associated inlet-side and outlet-side mountings and is perpendicular to the respective axis of symmetry.
In operation, the two flow tubes, which are commonly connected in parallel by means of a manifold at the inlet end and a manifold at the outlet end, vibrate as a pair of tuning fork tines with a phase difference of 180xc2x0, i.e., in phase opposition, whereby the laterally acting inertial forces of the two flow tubes cancel each other and are thus neutralized.
A major disadvantage of such double flow tube configurations is the use of the manifolds, which is inherent in the design. On the one hand, the manifolds present increased resistance to the moving fluid and constitute regions of the conduit in which deposits tend to build up. On the other hand, such manifolds, particularly if designed to reduce the aforementioned fluid-mechanical disadvantages, are expensive transducer components which account for a considerable part of the manufacturing costs of such Coriolis mass flowmeters.
The aforementioned disadvantages are largely eliminated in a transducer type as shown in U.S. Pat. No. 5,275,061, for example. This patent specification discloses an electromechanical transducer of the Coriolis type which provides an output in response to the mass flow rate of a fluid flowing in a pipe and which comprises:
a flow tube, curved in a tube plane symmetrically with respect to an axis of symmetry lying in this plane, for-conducting the fluid;
a rigid support body for mounting the flow tube,
the flow tube being fixed to the support body at an inlet end and an outlet end;
a rigid vibration isolator attached to the flow tube for forming a rigid, curved flow tube segment of a predeterminable three-dimensional shape which is dimensionally stable in operation; and
an excitation system which in operation excites the flow tube into vibrations in a mode symmetrical with respect to the tube plane, in which mode the three-dimensional shape of the flow tube segment is always preserved.
With fluid flow present, Coriolis forces cause the flow tube, excited in the manner described above, to be deformed antisymmetrically in the tube plane. The vibrations of the flow tube are sensed at the inlet and outlet ends, and the resulting phase-shifted sensor signals are suitably processed and evaluated.
It has turned out, however, that although virtually no torsional vibrations are produced by the aforementioned transducer, and the Coriolis-force-generating vibrations of the single flow tube, which are generally also those with the greatest amplitudes, are substantially neutralized in a simple manner by a tube shape that is variable in the tube plane but always remains symmetrical, the deformation movements of the flow tube and the resulting mass acceleration may cause inertial forces in the transducer which act in the direction of the axis of symmetry and, thus, in a direction transverse to the pipe. These inertial forces may cause undesired flexural vibrations of the connected pipe.
Furthermore, a vibration isolator of the kind described, which, as also proposed in U.S. Pat. No. 5,275,061, may also be designed as an enclosure of the flow tube segment, represents a cantilever mass which, when accelerated during oscillating motions of the excited and fluid-carrying flow tube, may also have a disturbing effect on the pipe. In addition, such a vibration isolator is a component that has to be manufactured additionally and incorporated into the transducer using additional process steps, and thus increases the manufacturing costs.
It is therefore an object of the invention to provide a transducer suitable for use in a Coriolis mass flowmeter which is easy to manufacture, permits the use of materials with a high modulus of elasticity and/or a high coefficient of expansion, particularly of high-quality steel, for the flow tube, and during operation causes virtually no or only very slight undesired vibrations, particularly no torsional vibrations and/or no flexural vibrations, in the connected pipe.
To attain this object, the invention consists in an electromechanical transducer of the Coriolis type which provides an output in response to the mass flow rate of a fluid flowing in a pipe and which comprises:
a flow tube, curved symmetrically with respect to an axis of symmetry lying in a tube plane, for conducting the fluid;
a rigid support body for mounting the flow tube,
the flow tube being fixed to the support body at an inlet end and an outlet end; and
an excitation system which in operation excites the flow tube into vibrations in a first eigenmode which is symmetrical in the tube plane.
In a first preferred embodiment of the invention, the flow tube is curved in the tube plane trapezoidally.
In a second preferred embodiment of the invention, the excitation system generates an excitation force which deforms the flow tube and acts in the direction of the axis of symmetry.
In a third preferred embodiment of the invention, the first eigenmode of the flow tube has at least three antinodes.
In a fourth preferred embodiment of the invention, the flow tube performs vibrations in an f3 natural mode.
A fundamental idea of the invention is to generate Coriolis forces in the flow tube, which is curved as described above, by exciting the latter into vibrations in the tube plane in such a dynamically balanced manner that inertial forces acting in the tube plane in the direction of the axis of symmetry are compensated for and thus substantially eliminated.
The principal advantages of the transducer according to the invention are that it can be made very compact and, because of the dynamic vibration isolation, very light while being easy to manufacture.
A further advantage of the invention is that both additional vibration isolators of the above-described kind and the parallel double flow tube configuration, and thus the costly-to-make manifolds, can be dispensed with.